1. Field of the Invention
The present invention relates to a shutter assembly for a camera having shutter blades which consist of metallized plastic.
2. Description of the Prior Art
Quite recently along with advances in electronification of camera, the shutter driving thereof has been changed from the conventional mechanical spring driving type into an electromagnetic driving source. Thus, along with advances in automatic cameras the electronically driven shutter is more frequently used, whereby various kinds of electromagnetically driven shutters whose blades are driven by means of an electromagnetic driving source have been proposed. However, in the case where the shutter is electromagnetically driven, there remain many problems to be solved. With regard to the capacity of the power source to be built into the camera it is desired to develop a battery which is compact and light and which has a large capacity. Because the battery must be small to enhance portability, there is a limit to the capacity of the power source which can be built in. The permanent magnet presently considered best for obtaining the magnetic power for the electromagnetic driving source is the rare earth magnet. However, it is still difficult to obtain a strong electromagnetic driving force due to the restrictions of the compact camera. Namely, it is difficult to obtain as high a shutter running speed as can be obtained by means of the conventional mechanical driving devices. Further, it is impossible to allow the field coil or the yoke constituting the electromagnetic driving source to be too large. On the other hand, the size is restricted due to the compactness of the camera.
Thus, in order to obtain a shutter running speed almost equal to that of the conventional mechanical shutter in a device using an electromagnetically driven shutter having properly selected magnetic circuit members such as the magnet, the coil and the yoke constituting the electromagnetic driving source which are combined with a compact built-in power source, it is proposed to lower the load for example by making the shutter blades themselves lighter in weight.
The above-mentioned problem is particularly important in a focal plane shutter wherein it is necessary to open and close an aperture with a comparatively large area, so that thus far shutter blades have been made of thin metal sheet small in specific weight. Structurally, the shutter blades are constituted of thin material (0.03-0.05 mm in thickness) for example if titan, aluminum, berylium and so on. Further, instead of thin metal sheet, light shading plastic sheet has been proposed as the material for the shutter blades.
The shutter blade member made of thin metal sheet which is small in specific weight is found to involved high cost without providing significant improvement in the running shutter speed. On the other hand, if the sheet is made thinner, the rolling cost increases further, pin holes occur more often, and there is a danger of deformation and rigidity. Further, in case of the light shading plastic, the light shading ability of the shutter blade is insufficient. Further, embossing is necessary in order to avoid projection of the calked portion for mounting the driving arms on the shutter blades. Thus the plastic sheet is hard to cold form so that the forming work is difficult. Also, under the higher temperature the formed part is apt to resume the original shape. Further, because the rigidity of the material is weak strong calking cannot be effected. Further, in case the shutter blades are arranged in the neighborhood of the focal plane of the camera as in the case of a focal plane shutter, the shutter blades are instantaneously burnt so as to cause a pin hole where an intense incident light coming through the lens froms an image on the shutter blades. The tests have proved that the pin hole is produced in a remarkably short time as compared with the case of a cloth shutter.
Consequently, it has been proposed quite recently to apply a thin metal sheet on the plastic sheet or metallize the surface of the plastic sheet.
However, when a thin metal sheet is applied upon the plastic sheet it is necessary to roll the metal sheet very thin, whereby the rolling cost is increased and there occurs greater danger of a pin hole. The smallest thickness which can be obtained for the metal sheet is 0.01-0.02 mm so that the overall thickness of the shutter blade with the metal sheet thereon cannot be made sufficiently small and therefore the running shutter speed cannot be significantly improved.
Because on the other hand, the thickness of the metal coating on the surface of the plastic sheet is as small as 0.0001-0.0003 mm, the overall thickness of a shutter blade can be made sufficiently small that the running shutter speed can be significantly improved. Namely, the metal coated plastic sheet can be made use of as shutter plane very effectively. However, such a plastic sheet is not suitable for constituting a plural number of the shutter blades. In order to make a shutter blade it is necessary to emboss a part of the shutter blade so that the calked portion is buried in the embossed part. Thus, the thickness of the metal coating on the plastic sheet is too small to obtain a high forming accuracy by embossing, while the rigidity of the calked portion is so weak that the shutter blade is inferior in durability and reliability.